Json parser fix. BUGZ-4225

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This merge request is automatically cherrypicked from !1838 (merged) from source branch https://source.coderefinery.org/aleksandr.konstantinov/arc/-/tree/json_parse_fix orginally made by @aleksandr.konstantinov .
The original MR went to branch: https://source.coderefinery.org/nordugrid/arc/-/tree/next
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Fixed Json processiong in REST interface. Added placeholder for unit tests to implement later.